The invention relates to an additional source of information about aircraft orientation provided to a pilot's auditory senses.
One cause of aviation accidents is spatial disorientation. Spatial disorientation occurs when pilots receive conflicting or misleading information from their visual vestibular and proprioceptive sensory systems. This may cause them to become confused about their physical orientation relative to the earth. This conflicting information is often so compelling that it causes pilots to question or even ignore their flight instrumentation and choose instead to fly on the basis of their own intuition about the true orientation of the aircraft. Sadly, such decisions may lead to tragic consequences.
Many spatial disorientation accidents are the result of vestibular illusions that cause the pilot to feel that the plane is in a different orientation than it actually is. For example, the oculogravic illusion is one common vestibular illusion that results when there is a change in linear acceleration. Linear acceleration produces a pitch up illusion while linear deceleration produces a pitch down illusion. This can result in a fatal accident when after a night take off and while still accelerating, the pilot falsely senses an excessive pitch angle and compensates with an unnecessary pitch down stick input resulting in impact with the ground.
The “leans” is another common vestibular illusion that is occurs when the pilot is in a prolonged turn. During the turn, the vestibular organs adapt to the point that they register the angle of bank used during the turn as being vertical. When the plane rolls to wings level to terminate the turn the pilot may perceive this rotation as a bank and turn in the opposite direction. This may cause pilots to lean in an attempt to assume what they think is a vertical posture. The leans may also occur when the pilot performs very slow roll to the left that does not stimulate the vestibular apparatus and then rolls rapidly to the right to level flight. Such a maneuver may generate the false impression that the plane has only rolled to the right.
One way to counter the effects of spatial orientation may be to provide pilots with an additional source of information about aircraft orientation that does not depend exclusively on the visual artificial horizon indicator currently used in a typical aircraft cockpit. Preferably, this redundant information should be presented to a non-visual modality to counter the visual-vestibular illusions such as the oculogravic illusion and the oculogyric illusions that can impair a person s ability to interpret visual information while under linear or rotational acceleration. Consequently, a number of researchers have proposed the use of a rudimentary artificial auditory artificial horizon to provide pilots with information about the pitch and roll of the aircraft through the manipulation of an auditory signal presented to the pilot through stereo headphones.
De Florez in “True Blind Flight” published in the Journal of the Aeronautical Sciences, 3, 168-170, 1936, used a continuous tone signal presented over headphones to provide two orientation cues to the pilot. The rate of turn of the aircraft was indicated by increasing the level of the tone in one ear and delaying the phase of the tone in the opposite ear thus changing the apparent left right location of the tone. The airspeed of the aircraft was indicated by increasing or decreasing the pitch of the tone. These audio cues were somewhat successful and the investigators reported that it was possible to fly an aircraft in a stable manner for more than 40 minutes while blindfolded solely on the basis of these cues.
However, they also noted that these tone based cues were fatiguing to the ear and suggested that a better alternative might be to base the cues on a broadcast radio signal that might be more appealing for the pilot to listen to for long periods of time. They suggested using interaural amplitude cues to manipulate the apparent left right position of the broadcast radio signal to indicate the rate of turn of the aircraft and using low frequency amplitude modulations of the radio signal to convey information about the rate of climb or dive of the aircraft.
T. W. Forbes at Harvard University conducted a number of experiments further exploring the use of an auditory attitude indicator in flight. One configuration published in The Journal of the Aeronautical Sciences, “Auditory Signals for Instrument Flying,” 13, 255-258, 1946 involved a three in one sound source that used a repetitive left right sweeping sound to indicate rate of turn. It also used a variation in the pitch of the tone to indicate bank angle and a variation in the interruption rate of the tone, causing a putt-putt like sound to indicate the airspeed of the aircraft. This configuration was not tested in flight but it was shown that it may be used to maintain a level flight pattern in a ground based trainer.
Lyons, Gillingham, Teas and Ercoline described an Auditory Orientation Instrument (AOI) in “The Effects of Acoustic Orientation Cues on Instrumant Flight Performance in a Flight Simulator,” Aviation, Space and Environmental Medicine, 1990, 61, 699-706. In one example, the AOI provided acoustic representations of three flight parameters. The first was airspeed, which was represented by the frequency of a square wave signal that increased with increasing velocity. Second was bank angle, which was indicated by a left right intensity panning of the sound. Third was vertical velocity, which was indicated by amplitude and modulating the envelope of the square wave that repeated crescendos indicating an increase in altitude and repeated decrescendos indicating a decrease in altitude. These audio cues were found to increase the pilot s ability to maintain a steady airspeed altitude and bank angle when no visual cues were present but not up the level of performance achieved when visual cues were available.
More recently Grohn, Lokki, and Takala (published in Proceedings of the International Conference on Auditory Display (ICAD), Syndey Australia Jul. 6-9, 2004 (Grohn, Lokki, and Takala) have discussed the use of an auditory attitude indicator for maneuvering through a virtual environment with a 6-degree of freedom flight model. This attitude indicator was based on a 3-D audio display that used Head-Related Transfer Functions (HRTFs) to manipulate the apparent locations of sounds presented to the listener over headphones. In order to determine where to place the virtual sound in their attitude display Grohn Lokki and Takala relied on what they called a ball on a plate metaphor. In this metaphor, the apparent direction of the sound source was determined by the downward direction a ball would roll if it were located on a plate with the same attitude orientation as the operator's vehicle.
in one example, Grohn, Lokki, and Takala reported using three additional cues to provide the operator with additional information about the amount of tilt in the aircraft attitude. These include a gain cue, where the level of the pulsed pink noise increased with the amount of tilt and the pulsed noise was inaudible when the operator was level. A pitch cue, where a narrow band noise was added to the stimulus with the center frequency of the noise varying from 50 Hz to 2 kHz as the amount of tilt increased. A rate cue where the pulse rate of noise increased from 0.7 Hz when the operator was level to 8 Hz when the operator was fully tilted. These three conditions were tested in a virtual flight task. All three were found to result in lower pitch and roll errors than those obtained in a visual only control condition with the same visual cues and but no auditory horizon cue. Minimal difference in performance was found between the three audio conditions but the gain cue condition was preferred because it was the only one where the pulsed noise sound disappeared when the operator was in a level orientation. The subjects considered the other conditions to be annoying because they resulted in pulsed noise sounds even when the operator was flying straight and level.
The previously described systems have not adequately addressed the requirements necessary to make an auditory attitude indicator practical for everyday use in actual aircraft. In one example, Grohn, Lokki, and Takala provided a pitch cue that was based on the spectral elevation cues that naturally occur in the Head Related Transfer Functions (HRTFs) of human listeners. Specifically, their system used HRTFs to place a virtual sound source in the direction that a ball would roll if it were tilted with the same azimuth as the aircraft. Thus, a downward pitch would result in a virtual sound source located in front of the listener and an upward pitch would result in a sound source located behind the listener. However, prior research has shown that it is very difficult to distinguish between sound sources located at the same lateral angle in front and behind the head. Such sources produce nearly identical binaural cues and thus are said to fall on the cone of confusion with respect to the listener. This is discussed in “The role of head movements and vestibular and visual cues in sound localization,” The Journal of Experimental Psychology, 27, 339-368, 1940. Thus, the listener might easily confuse upward and downward pitch angles in the Grohn, Lokki, and Takala system.
All documentation referenced within this application is herein incorporated by reference.